Acidic deposition in the northeastern United States: Sources and inputs, ecosystem effects, and management strategies

North America and Europe are in the midst of a large-scale experiment. Sulfuric and nitric acids have acidified soils, lakes, and streams, thereby stressing or killing terrestrial and aquatic biota. It is therefore critical to measure and to understand the recovery of complex ecosystems in response to decreases in acidic deposition. Fortunately, the NADP, CASTNet, and AIRMoN-dry networks are in place to measure anticipated improvements in air quality and in atmospheric deposition. Unfortunately, networks to measure changes in water quality are sparse, and networks to monitor soil, vegetation, and fish responses are even more limited. There is an acute need to assess the response of these resources to decreases in acid loading. It would be particularly valuable to assess the recovery of aquatic biota - which respond directly to acid stress - to changes in surface water chemistry (Gunn and Mills 1998). We used long-term research from the HBEF and other sites across the northeastern United States to synthesize data on the effects of acidic deposition and to assess ecosystem responses to reductions in emissions. On the basis of existing data, it is clear that in the northeastern United States • reductions of SO₂ emissions since 1970 have resulted in statistically significant decreases in SO₄^2- in wet and bulk deposition and in surface waters • emissions of NO_X and concentrations of NO₃^- in wet and bulk deposition and in surface waters have shown no increase or decrease since the 1980s • estimates of NH₃ emissions are uncertain, although atmospheric deposition of NH₄⁺ remains important for forest management and stream NO₃^- loss • acidic deposition has accelerated the leaching of base cations from soils, thus delaying the recovery of ANC in lakes and streams from decreased emissions of SO₂ (at the HBEF the available soil Ca pool appears to have declined 50% over the past 50 years) •sulfur and N from atmospheric deposition have accumulated in forest soils across the region, and the slow release of these stored elements from soil has delayed the recovery of lakes and streams after emissions have been reduced • acidic deposition has increased the concentration of toxic forms of Al in soil waters, lakes, and streams • acidic deposition has leached cellular Ca from red spruce foliage, which has made trees susceptible to freezing injury and led to more than 50% mortality of canopy trees in some areas of the Northeast • deficiencies of Ca²⁺ and Mg²⁺ have caused extensive mortality of sugar maple in Pennsylvania, and acidic deposition contributed to the depletion of these cations from soil • forty-one percent of lakes in the Adirondack Mountains and 15% of lakes in New England have exhibited chronic or episodic acidification or both; 83% of the affected lakes are acidic because of atmospheric deposition • the ANC of surface waters in New England has increased only modestly, and the Adirondack and Catskill regions have experienced no significant improvement, after decreases in atmospheric S deposition in recent decades • acidification of surface waters has resulted in a decrease in the survival, size, and density of fish and in the loss of fish and other aquatic biota from lakes and streams • emissions of air pollutants have important linkages to other large-scale environmental problems, including coastal eutrophication, mercury contamination, visibility impairment, climate change, and tropospheric ozone Moreover, we anticipate that recovery from acidic deposition will be a complex, two-phase process in which chemical recovery precedes biological recovery. The time for biological recovery is better defined for aquatic than for terrestrial ecosystems. For acid-affected aquatic ecosystems, we expect that stream populations of macroinvertebrates and lake populations of zooplankton will recover 3-10 years after favorable chemical conditions are reestablished; recovery of fish populations would follow. For terrestrial ecosystems, trees would probably respond positively to favorable atmospheric and soil conditions only over decades. Indicators of chemical recovery (soil percentage base saturation, soil Ca:Al ion ratios, and surface water ANC) were used to evaluate ecosystem response to proposed policy changes in SO₂ emissions. Projections made using an acidification model (PnET-BGC) indicate that full implementation of the 1990 CAAA will not afford substantial chemical recovery at the HBEF and at many similar acid-sensitive locations. Although uncertainties remain, our analysis indicates that current regulations will not adequately achieve the desired outcomes of the 1990 CAAA. Those desired outcomes include greater ANC for lakes and streams, greater diversity and health of fish populations, and less degradation of forest soil and stress to trees (USEPA 1995). Model calculations indicate that the magnitude and rate of recovery from acidic deposition in the northeastern United States is directly proportional to the magnitude of emissions reductions. Model evaluations of policy proposals calling for additional reductions in utility SO₂ and NO_X emissions, year-round emissions controls, and early implementation (2005) indicate greater success in facilitating the recovery of sensitive ecosystems and in accomplishing the goals of the Clean Air Act than current 1990 CAAA targets could deliver. Note that improvements in the rate of acidic deposition from SO₂ controls on utilities may be offset by NO_X emissions unless transportation emissions of NO_X are curtailed. Specific targets for reducing emissions should be based on clear goals that meet the extent and schedule of recovery of sensitive aquatic and terrestrial ecosystems envisioned under the Clean Air Act.